Complexation of Thorium(IV) and Uranium(IV) by a Hexaacetic Hexaaza Macrocycle: Kinetic and Thermodynamic Topomers of Actinide Chelates with a Large Cavity Ligand

The solution behavior of the Th(IV) chelate with a polyaza polycarboxylic ligand, HEHA (1,4,7,10,13,16-hexaazacyclooctadecane-N,N ',N ",N "',N ",N "'-hexaacetic acid), is investigated by one- and two-dimensional NMR spectroscopy. [Th(HEHA)](2)(-) exhibits a very unusual irreversible topomerization process from a kinetically stable topomer of C(2) symmetry to a thermodynamically stable topomer of remarkably high symmetry (S(6)). The metal ion appears to be 12-coordinated in both geometries with a pseudoicosahedral arrangement of the chelating groups. The activation energy of the irreversible topomerization is exceedingly high (199 kJ.mol(-)(1)), and the slow kinetics of conversion is assigned to a complete reorganization of the chelate. U(IV) forms directly a stable entity of C(2) symmetry with HEHA and no reorganization of the complex is observed. The unusual topomerization phenomenon undergone by HEHA, is thus strongly dependant on the ionic radius of the encapsulated metal ion. The corresponding lanthanide chelates are totally asymmetric.     

Hydrodynamic properties and unperturbed dimensions of polycyclohexyl and polyphenyl methacrylates in various solvents

Intrinsic viscosity, sedimentation, light scattering and osmotic pressure measurements have been made on dilute solutions of polycyclohexyl (PCy) and of polyphenyl methacrylates (PPh). Relations between (η), (S₀), second virial coefficient and molecular weight have been established.     

Modification of calix[4]arenes with CMPO-functions at the wide rim. Synthesis, solution behavior, and separation of actinides from lanthanides

Two calix[4]arene tetraethers (Y = C5H11, C14H29) bearing on their wide rim four -N(Me)-CO-CH2-P(O)Ph2 residues were synthesized for the first time. Their ability to extract lanthanides and actinides from an acidic aqueous phase to organic phases (CH2Cl2, NPHE) was studied. In comparison to the corresponding -NH-analogs, they are less efficient extractants, the selectivity for the light over the heavy lanthanides is less pronounced, while there is still an interesting selectivity of Am3+ over Eu3+. Stability constants for selected lanthanide salts were determined also in homogenous phase (methanol, acetonitrile) but do not account for the different extraction results. The complexation of Gd3+ was also followed by relaxivity (NM RD) measurements, which suggest an even stronger aggregation for the N-methyl compound while the 1:1 complex is reached for a smaller ratio [L]/[Gd3+] compared to the NH analog. The formation of aggregates is also supported by dynamic light scattering measurements. A single crystal X-ray structure of the pentyl ether reveals a C2-symmetrical pinched cone conformation for the free ligand.     

Search for a fourth-generation quark more massive than the Z0 boson in p&pmacr; collisions at radicals = 1.8 TeV

We present the results of a search for pair production of a fourth-generation charge -1 / 3 quark (b(')) in sqrt[s] = 1.8 TeV p&pmacr; collisions using 88 pb(-1) of data obtained with the Collider Detector at Fermilab. We assume that both quarks decay via the flavor-changing neutral current process b(')-->bZ(0) and that the b(') mass is greater than m(Z)+m(b). We studied the decay mode b(')b(');-->Z(0)Z(0)b&bmacr; where one Z0 decays into e(+)e(-) or &mgr;(+)&mgr;(-) and the other decays hadronically, giving a signature of two leptons plus jets. An upper limit on the sigma(p&pmacr;-->b(')b(');)x[B(b(')-->bZ(0))](2) is established as a function of the b(') mass. We exclude at 95% confidence level a b(') quark with mass between 100 and 199 GeV/c(2) for B(b(')-->bZ(0)) = 100%.     

X-Ray waveguiding studies of ordering phenomena in confined fluids

We have determined the structure of a colloidal fluid confined in a gap between two walls by making use of the waveguiding properties of the gap at x-ray wavelengths. The method is based on an analysis of the coupling of waveguide modes induced by the density variations in the confined fluid. Studies on suspensions confined within gaps of a few hundred nanometers showed strongly selective mode coupling effects, indicative of an ordering of the colloidal particles in layers parallel to the confining walls.